This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The DNA genetic information in our cells is packaged as chromatin. The nucleosome core particle is a fundamental repeating unit of chromatin and comprises about 150 base pairs of DNA spooled around a histone protein core. Recent studies show that chromatin is not simply a repressive structure that occludes DNA, but is instead an active participant in gene regulation that associates with multiple chromatin factors and enzymes. Despite the importance of such interactions, we currently lack a structural understanding of how chromatin factors or enzymes recognize the nucleosome. To address this problem, we have reconstituted complexes containing the nucleosome core particle and several chromatin factors or enzymes, and we have grown single crystals of a chromatin factor/nucleosome core particle complex. We request beam time at APS for this challenging project which features large macromolecular complexes, weakly diffracting crystals and large unit cell dimensions.